The recent proliferation of, and resulting stiff competition among, wireless communications products have led to price/performance demands that are difficult to meet with conventional technologies. The demands placed on radio-frequency interconnections between printed wiring boards (PWBs), and between printed wiring boards and other devices, such as antennas and filters, are no exception. Such interconnections are usually made via coaxial connectors and cables. These interconnections are expensive in terms of both the cost of the parts and the costs of manufacturing and assembly.
An alternative way of making interconnections is via capacitive coupling. A capacitive coupler is formed by two metallic parts that are a part of, or are directly connected to, the transmission lines of the components that are to be coupled, and that are positioned in close proximity to each other. One typical configuration takes the form of gap-coupled microstrip lines, where two parallel conductors, formed on the same layer of a PWB, almost but not quite abut each other or extend for a distance past each other in close proximity, whereby the adjacent edges (ends or sides) of the two conductors form a capacitor. Another typical configuration takes the form of broadside-coupled microstrip lines, where two parallel conductors, formed on adjacent layers of a PWB, extend for a distance over each other, whereby the adjacent faces of the two conductors form a capacitor. The capacitance, and consequently the size of the parts forming the capacitor, must be large enough to provide a low-impedance connection. To increase the capacitance, the adjacent edges or faces of the two conductors may be made wider than the remainder of the two conductors. However, even at wireless radio-frequencies (e.g., 0.8-3.5 GHz), the required size of the parts that form the capacitor is prohibitively large for many applications. Moreover, the parts must be positioned very close together, and therefore cannot bridge a considerable distance, e.g., the distance between PWBs.